Application of an Electrorefiner Model to Identify Potential Sensors to Detect Off-Normal Operations

Pyroprocessing is being considered for application to treatment and recycling of spent nuclear fuel. During this process, a molten salt electrolyte comprised primarily of eutectic LiCl-KCl becomes progressively contaminated with metals from the spent fuel in chloride form. Normal operations would typically involve deposition of only uranium metal onto a cathode. But it has been hypothesized that intentional or unintentional neglect of the system can result in changes in the salt composition that could lead to co-deposition of uranium and plutonium. From a process control perspective, the operator would benefit from real time system feedback to indicate when this may occur. And from a safeguards perspective, an inspector could use logged system feedback to evaluate when this may have occurred. In this study, the ERAD model was used to simulate a scenario in which the molten salt electrolyte progressively changed from repeated processing of spent fuel without operator intervention. Since using a new batch of eutectic salt after each cathode deposition would be impractical and expensive in a commercial setting, the model used has been tailored to adjust the composition of the salt after each instance of waste reprocessing in order to represent a realistic scenario. Under certain sets of conditions, it was found that the cathode deposit would initially be pure uranium but would later transition to U/Pu co-deposition. This is likely due to the increasing Pu/U ratio in the salt. While it may not be possible to precisely track changes in the salt composition in real time, the model indicated that markers such as changes in electrode potentials or salt density can be detected that will indicate when Pu starts to deposit on the cathode. This ultimately should provide more options to inspectors who have limited access to data generated by the ERs and may have reason to doubt the fidelity of this data. Salt density, for example, can be determined during an on-site visit in which an inspector requests a sample and has it analyzed in a separate system controlled and maintained by the IAEA.